Evidence of Th2 polarization of the sentinel lymph node (SLN) in melanoma

Melanoma has a propensity for lymphatogenous metastasis. Improved understanding of the sentinel lymph node (SLN) immunological environment may improve outcomes. The immune phenotype of fresh melanoma SLNs (n = 13) were compared to fresh control lymph nodes (n = 13) using flow cytometry. RNA was isolated from CD4⁺ T cells of the SLN and control lymph node and assessed for Th1/Th2 gene expression pathways using qRT-PCR. In addition, VEGF expression was compared between primary melanoma (n = 6) and benign nevi (n = 6) using immunohistochemistry. Melanoma SLNs had fewer CD8⁺ T cells compared to controls (9.2% vs. 19.5%, p = 0.0005). The CD8⁺ T cells within the SLN appeared to have an exhausted phenotype demonstrated by increased PD-1 mRNA expression (2.2% vs. 0.8%, p = 0.004) and a five-fold increase in CTLA-4 mRNA expression. The SLN also contained an increased number of CD14 (22.7% vs. 7.7%, p = 0.009) and CD68 (9.3% vs. 2.7%, p = 0.001) macrophages, and CD20 B cells (31.1% vs. 20.7%, p = 0.008), suggesting chronic inflammation. RT-PCR demonstrated a significant Th2 bias within the SLN. In vitro studies demonstrated a similar Th2 polarization with VEGF treatment of control lymph nodes. The primary melanoma demonstrated strong VEGF expression and an increase in VEGFR1 within the SLN. Melanoma is associated with Th2-mediated "chronic inflammation," fewer cytotoxic T cells, and an exhausted T cell phenotype within the SLN combined with VEGF overproduction by the primary melanoma. These immunologic changes precede nodal metastasis and suggests consideration of VEGF inhibitors in future immunotherapy studies.